How do I change the approval status of a timesheet (i.e. I rejected the timesheet but I should have approved it)

Once a timesheet is rejected there is no way to recall it. It will go back to the contractor who will be notified via Email. They will then need to re-enter the time. The timesheet will have the original submission data there so they can simply resubmit. If changes are required, then they can amend accordingly.